This invention relates generally to electrical stepping motors, and more particularly to a micro-miniature stepping motor suitable for use in an electronic watch to drive a gear works for turning the time-indicating hands.
A stepping motor is one that rotates by way of short, essentially uniform angular movements rather than continuously. Typically, the steps are in 30.degree., 45.degree. or 90.degree. increments. The angular stepping action is obtained electromagnetically rather than by a ratchet and pawl mechanism as in a stepping relay.
An important application of the stepping motor is in electronic watches having an analog display formed by rotating hands. In an electronic watch of this type, low-frequency timing pulses are derived by frequency division from a high-frequency crystal-controlled time base. These timing pulses serve to actuate a stepping motor which drives the gear works rotating the hands of the watch.
In the context of an electronic watch, the stepping motor must not only be in micro-miniature form to minimize space requirements, but its power consumption must be extremely low so as to conserve power. In order to energize the watch with a miniature power cell which will last at least a year, the allowable power consumption is usually less than 8 microwatts.
Various types of stepping motors are known which make use of a permanent magnet rotor that cooperates with one or more stator coils to which actuating pulses are applied. thus in the prior U.S. Pat. No. 3,731,125 of Nickaldo, the rotor is formed by a permanent magnet disc that is slotted to define a circular series of magnet poles of alternate polarity. A pawl and detent is necessary in this motor to prevent backward motion of the rotor. Similarly, in the U.S. Pat. No. 3,818,690 to Schwartzschild, a pulsed stepping motor incorporates a mechanical detent for holding the rotor in a stable position, the rotor being advanced from one stable position to another by d-c pulses whose duration is sufficient to overcome the detent action.
The use of a mechanical detent in a stepping motor not only complicates and enlarges its physical structure, but it also imposes heavier power requirements thereon. Moreover, the use of a detent introduces structural asymmetries which render the motor more sensitive to external accelerations.
It has been recognized that a critical factor involved in the design of micro-miniature stepping motors for watches worn on the wrist is the sensitivity of the motor to external acceleration forces resulting from wrist movement. These movements at times may be be very rapid and abrupt. On one hand, it is desirable that the motor consume as little power as possible consistent with the requirement that sufficient torque be produced to drive the gear train and whatever other mechanisms are included in the watch, such as a calendar mechanism. On the other hand, it is important that the motor should not be affected by shock and other external accelerative forces that tend to advance the gear train, for then inaccuracies will be introduced in the timing.
The moment of inertia of a body, such as a stepping motor, depends upon the distribution of the masses of its component parts. If the distribution is asymmetrical, then an acceleration force gives rise to a torque that may be sufficient to turn the rotor. One can prevent external acceleration forces from turning the rotor of a stepping motor by so detenting the motor that these forces are insufficient to overcome the detent action. But the usual detent for this purpose has two disadvantages. First, the detent may itself introduce asymmetry, rendering the motor more sensitive to acceleration forces. Second, the strength of the detent is such that in order to diminish the sensitivity of the motor to acceleration forces, it requires a measurably greater input power to overcome the detent in order to step the rotor.